OXOMOLYBDENUM(V) IRON(III) PORPHYRINATE COMPLEXES - EFFECT OF AXIAL LIGAND PLANE ORIENTATION ON COMPLEX STABILITY, REDUCTION POTENTIAL, ANDNMR AND EPR-SPECTRA/

The compounds henyl]porphyrinato}bis(2-methylimidazole)iron(III) chlor ide, Fe(2,3-Mo-TTP)(2MeImH)(2)Cl (1), and enyl]porphyrinato]-bis(2-met hylimidazole)iron(III) chloride, Fe(3,4-Mo-TTP)(2MeImH)(2)Cl) (2), hav e been prepared in order to assess the effect of axial ligand plane or ientation upon the stability, reduction potential, and NMR and EPR spe ctra of these novel (porphryinato)iron(III)-Mo(V) systems that possess two S = 1/2 metal centers. The proton NMR spectra of 1 and 2 are char acteristic of perpendicular orientation of the planes of the axial 2Me ImH ligands. These results contrast with those previously reported (Ba su, P.; Shokirev, N. V.; Enemark, J. H.; Walker, F. A. J. Am. Chem. Se c. 1995, 117, 9042-9055) for the analogous compounds with NMeIm as the axial base (3, 4) whose H-1 NMR spectra are characteristic of one or both axial ligands in parallel planes. The equilibrium constants (beta (2)) for binding the bulky 2MeImH ligands of 1 and 2 are more than an order of magnitude smaller than those for NMeIm binding to 3 and 4. Th ree distinct pseudo-Nernstian one-electron couples are observed for 1 and 2 in DMF that can be assigned to the Fe(mm), Mo(V/IV), and Fe(II/I ) reductions, with the Fe(III/II) couple being most positive. The Fe(m m) and Mo(V/IV) potentials are similar to those for 3 and 4 and only s lightly perturbed from those of the individual isolated components. Th e EPR spectrum of 1 shows features due to Mo(V) and low-spin Fe(III) t hat are perturbed by weak exchange coupling (2.6 GHz, 0.078 cm(-1)) be tween the two metal centers which are separated by similar to 7.9 Angs trom. The ''large g(max)'' feature characteristic of the 2MeImH adduct s of Fe(III) tetraphenylporphyrinates is shifted toward the Mo(V) sign al to 2.85; the anisotropy of the Mo(V) signal is lost, and no molybde num hyperfine can be detected. The EPR spectrum of 2, which has a meta l-metal separation of similar to 9.4 Angstrom, shows an unperturbed '' large g(max)'' value of 3.41 for the Fe(III) center. The Mo(V) part of the spectrum is slightly perturbed from that of the precursor catecho late complex but is essentially identical to that of 4, which exhibits a rhombic Fe(III) signal.